The specification of genes which control the development of muscle cells and the ensuing assembly of myofibrillar structures characteristic of the differentiated state of muscle is the central theme of this proposal. The production and isolation of large numbers of mutants specifically affecting the structure and function of body muscle cells in the nematode, Caenorhabditis elegans, will provide the experimental foundation for this approach. The genetical analysis of these mutants will be pursued in order to understand the chromosomal arrangement of genetic elements involved in muscle cell development. Structural studies of normal and mutant muscles using light and electron microscopic methods will help in dissecting the effects of genetic alteration upon cellular, subcellular and supramolecular organization. The embryonic, larval and adult stages of normal and mutant nematodes will be studied on these several levels in order to determine the action of genes affecting muscle with respect to developmental time. Temperature-sensitive mutants may be particularly useful for this purpose.